Smart synchronization method of a web inspection system

ABSTRACT

A smart synchronizing method of a web inspection system for monitoring a moving web. A synchronizing device, at least one slave camera, and at least one lighting device illuminate an area of the web arranged to be imaged by the cameras. The method includes transmitting a synchronizing signal to the slave camera. The synchronizing signal includes at least a start pulse and serial data including additional information. All the cameras are synchronized with each other based on synchronization moment when integration of cameras end. A light synchronizing signal is transmitted to the at least one lighting device indicating a switching on and off times of the at least one lighting device, which switching off time corresponds to the synchronization moment, and calculating a starting time of integration based on an individual integration time of a camera and the synchronization moment common for cameras of the web inspection system.

TECHNICAL FIELD

The aspects of the disclosed embodiments relate to a method forsynchronizing cameras of a web inspection system monitoring moving webof continuous manufacturing processes.

The aspects of the disclosed embodiments also relate to a web inspectionsystem and a computer program product causing a web inspection system tocarry out the method.

BACKGROUND

In continuous manufacturing processes, there are materials or productsconstantly running through the machine. In such processes, for example,in paper machines, the product must be monitored in order to detectpossible deviations, and to obtain a final product of high quality, forexample, by web inspection systems such as camera systems comprising aplurality of area scan (array) cameras containing a large matrix ofpixels that capture a 2D image of a given area in one exposure cyclewith horizontal and vertical elements, for example in 640×480 pixelsi.e. with an area scan camera, a matrix of pixels provides an image ofthe monitored target. As the monitored target moves past the area scancamera system, images of it are exposed by an area image sensor of thecamera and transferred to a processing unit for analysis.

If there are more than one camera in the monitoring system, they need tobe synchronized so that they always take a picture of the subject atexactly the same time and in such a way that the results and images ofthe adjacent cameras can be combined when results are processed in thelater stage. Due to the camera technology, the switching off of thepulsed lights also needs to be precisely synchronized with the end ofthe camera integration.

Usually, the cameras are synchronized with each other by a simplesynchronizing signal that defines an image capturing time for cameras.This kind of synchronization is adequate provided that a fixed framerate is used. But, however, when the simple synchronizing signal thatonly defines an image capturing time for cameras is used, the cameras donot know exactly the image number of the other cameras or the distancethat the web has traveled, and therefore, combining the results andadjacent images at a later stage is difficult or almost impossible.Furthermore, if the pulsed light is not switched off exactly at the sametime with the end of the camera integration, the integration of pixelswill continue, which may lead to a poor quality of image, for example,the image may be too bright.

CN204287062U discloses a synchronizing method of a web inspection systemfor monitoring a moving web and comprising a master camera synchronizingslave cameras and lighting devices. EP3343896A1 discloses a method andsystem for synchronizing active illumination pulses in a multi-sensorimager.

SUMMARY

It is an aim of the aspects of the disclosed embodiments to provide andpresent a novel method for synchronizing cameras and at least onelighting device of a web inspection system monitoring moving web ofcontinuous manufacturing processes. The further aim is to provide a webinspection system and a computer program product causing the webinspection system to carry out the method. The synchronizing method,system, and computer program product according to the aspects of thedisclosed embodiments are characterized in what will be presented in theindependent claims, and the dependent claims relate to advantageousembodiments of the present disclosure.

According to a first aspect, there is provided a smart synchronizingmethod of a web inspection system for monitoring a moving web andcomprising a synchronizing device and at least one slave camera, and atleast one lighting device arranged to illuminate an area of the webarranged to be imaged by the cameras, the method comprising:transmitting a synchronizing signal to the at least one slave camera bythe synchronizing device, wherein the synchronizing signal comprises atleast a start pulse and serial data comprising additional informationfor the at least one slave camera, and wherein all the cameras of theweb inspection system are configured to be synchronized with each otherbased on the synchronizing pulse indicating the synchronization momentwhen integration of cameras end, transmitting a light synchronizingsignal to the at least one lighting device by the synchronizing device,wherein a light controlling pulse of the synchronizing signal indicatesa switching on and off times of the at least one lighting device, whichswitching off time corresponds to the synchronization moment, andcalculating a starting time of integration based on an individualintegration time of a camera and the synchronization moment common forcameras of the web inspection system.

According to an example, the method further comprises startingintegration of the cameras at their calculated starting times ofintegration and switching on the at least one lighting device forilluminating the area on the web imaged by the cameras, and endingintegration of all cameras and switching off the at least one lightingdevice at the time of synchronization moment. According to an example,the method further comprises reading intensity values from pixels ofcameras measured during integration and resetting pixels, formingcaptured image data based on the read intensity values, and transmittingthe captured image data with received additional information to an imagedata processing device. According to an example, the method furthercomprises forming a combined image from the received image data usingthe received additional information by the image data processing device.According to an example, all cameras of the web inspection system arearea scan cameras. According to an example, the synchronizing device isa smart camera that is one of the cameras of the web inspection system.According to an example, the synchronizing device is an externalsynchronizing device.

According to a second aspect, there is provided a web inspection systemfor monitoring a moving web and comprising a synchronizing device and atleast one slave camera, at least one lighting device, and an image dataprocessing device, wherein the cameras are synchronized with each otherby using the method according to a first aspect and its examples.

According to an example, the at least one slave camera is an area scancamera. According to an example, the cameras of the web inspectionsystem are arranged adjacently as a row. According to an example, thesynchronizing device is a smart camera of the web inspection system.According to an example, the synchronizing device is an externalsynchronizing device. According to an example, the web inspection systemfurther comprises an edge marking device.

According to a third aspect, there is provided a computer programproduct, stored on a computer readable medium and executable in acomputing device, wherein the computer program product comprisesinstructions for a synchronizing device of a web inspection systemcomprising at least one slave camera and at least one lighting device toperform the method according to a first aspect and its examples.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the aspects of the disclosed embodiments will bedescribed in more detail with reference to the appended drawings, inwhich

FIG. 1 a shows a web inspection system comprising a group of adjacentcameras and a lighting device that are synchronized with each other byusing the smart synchronization method according to an embodiment of thepresent disclosure,

FIGS. 1 b-e show images captured by cameras of the web inspection systemof FIG. 1 a and combined images of these images,

FIGS. 1 f-g show images captured by cameras of a web inspection systemcomprising adjacent cameras but not using the synchronization methodaccording to an embodiment of the present disclosure,

FIG. 2 shows smart synchronization signals of a smart synchronizationmethod of a web inspection system according to an embodiment of thepresent disclosure,

FIG. 3 shows a web inspection system using a smart synchronizationmethod according to an embodiment of the present disclosure forsynchronizing its cameras and lighting devices, and

FIG. 4 shows a block diagram of a smart synchronization method of a webinspection system according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Several adjacent image sensors that are area scan cameras also called asmatrix cameras of a web inspection system monitor a moving web and eachof them is arranged to capture images one image frame, i.e. one imagematrix at a time as the web being inspected moves past the field ofviews of cameras. The captured adjacent images cover substantially thewhole width of the web. Each image is captured by exposing atwo-dimensional array of photosensitive elements for a brief period,called the integration or shutter time, to light that has been focusedon the array by a lens. The array may be called an image and theindividual photosensitive elements may be called pixels. Each pixelmeasures the intensity of light falling on it during integration. Afterthe integration ends, the measured intensity values are read, the pixelsare reset i.e. short-circuited to black level until the next integrationstarts. The measured analogy intensity values are then converted todigital information that is then processed to form the actual capturedimage. The integration is delimited by the behaviour of the readoutelectronics and is independent of the exposure made by the shutter. Theweb to be monitored may be, for example, the web-like material of apaper or cardboard machine, or some other kind of machine comprisingmoving parts to be monitored, such as a printing machine comprising amoving roll, or an assembly machine.

Image data of each captured image may then be transmitted to an imagedata processing device for processing. The image data processing devicemay be, for example, a computer, a logic circuit, a digital signalprocessor (DSP) or an image data computer or any other computing devicesuitable for processing images of this type. For processing adjacentimages may be combined as a single image with enlarged image field andfor this combining, it is important that images of the combined imageare captured so that at the time of capturing the web has travelled thesame distance in the moving direction in all images so that adjacentimages are parallel and aligned to each other and thus form alongitudinal rectangular combined image perpendicular to the movingdirection of the web when combined.

Simple synchronizing pulses, provided by a master camera or an externalmodule, may be used for synchronizing adjacent cameras and lightingdevices of a web inspection system with each other by indicating a firsttime point when to start image capturing (start integration of pixels)and when to switch lights on and a second time point when to end imagecapturing (end integration of pixels) and when to switch lights off.Integration time is dependent on the time interval the lighting devicesare switched on. However, this kind of simple start pulse is notadequate in varying situations, for example when there is need to changethe capturing frequency (frame rate) on the fly but may work when imagecapturing frequency of cameras are predetermined and remain the same.Furthermore, when this kind of simple synchronizing pulse is used,cameras do not add in connection with captured images informationrelating, for example, to the image sequence number and/or to thedistance that the web has travelled in the moving direction at the timeof image capturing and nor is such information otherwise available,although such information is needed when images are combined. Therefore,these images are very hard or at least sometimes even impossible to becombined as a single combined image. Furthermore, it is also possiblethat captured images are not substantially parallel and if an event iscomprised at least by one of the images and a defect map is formed basedon these non-parallel images, the defect map cannot be reliably used forlocating and/or correcting the defect in the web.

Therefore, in the present disclosure a smart synchronizing method isused for synchronizing a web inspection system having more than oneadjacent camera instead of simple synchronizing pulse. In the smartsynchronizing method a master camera, that may be one of the cameras ofthe web inspection system, is used for synchronizing other cameras ofthe web inspection system that are so called slave cameras and at leastone lighting device or an external synchronization device may be usedfor synchronizing cameras of the web inspection system and at least onelighting device. If the external synchronization device is used forsynchronizing, all the cameras are slave cameras. Whether the devicetransmitting the synchronizing signal is either a master camera or anexternal device, it may be called aa a synchronizing device.

The master camera synchronizes the slave cameras by transmitting them asynchronizing signal. And in a case of the external synchronizationdevice, it is the external synchronization device that transmit thesynchronizing signal to cameras of the web inspection system. Thesynchronizing signal indicates a synchronizing moment for cameras. Thesynchronizing moment corresponds a switching off time of lights of atleast one lighting device. The cameras define starting of theirintegration so that it ends simultaneously with the transmittedswitching off time of lights and because integration of all cameras endsat the same time and also lights are switched off at the same time,adjacent images are taken of the web at the same time i.e.simultaneously. Further, the synchronizing signal may comprise serialdata field comprising additional information transmitted that may beused when combining of images of adjacent cameras or controlling camerasettings. This additional information easies the combination of theseimages and thus they can be reliably combined with each other forprocessing stage, for example, for analyzing, and/or for forming adefect map. In the smart synchronizing method, for synchronizing the atleast one lighting device of the web inspection system with cameras thesynchronizing device transmits a light synchronizing signal to the atleast one lighting device. A light controlling pulse of the lightsynchronizing signal indicates a switching on and off times of at leastone lighting device. The smart synchronization method comprisestransmitting both signals, the synchronizing signal to the at least twocameras and a light synchronizing signal to the at least one lightingdevice.

The web inspection system may further comprise an edge marking device.The smart camera or the external synchronization unit may be used forcontrolling the edge marking device. The edge marking device sprays amarking in the edge are of the paper web. The marking may be, forexample, so called moving direction (MD)-marking, which is sprayed ontothe web at certain intervals. The interval may be, for example, 25 m,100 m or 1000 m or any other predetermined distance. And based on themarking, a defect is easier to be located for correction. The markingmay be visible or invisible for eyes, but visible for UV-light, it maybe dot or line code, for example, a certain type of binary code.

FIG. 1 a shows a web inspection system 10 comprising a group of cameras11, 12, 13, 14 and a lighting device 16 that are synchronized with eachother by using a smart synchronization method according to an embodimentof the present disclosure. The smart synchronization method comprisestransmitting of a synchronizing signal and a light synchronizing signal.The web inspection system 10 comprises 4 adjacent cameras 11, 12, 13,14. The lighting device 16 is configured to illuminate field of views ofthe cameras 11, 12, 13, 14 on an inspected web 15. All cameras 11, 12,13, 14 are area scan cameras that see different parts of the web. One ofthe cameras, the camera 11 in this embodiment, is a master camera andthe other cameras 12, 13, 14 are slave cameras. Each camera 11, 12, 13,14 has its own integration time that it predetermined for it or which ithas automatically controlled/adjusted based on intensity of itspreviously captured image(s). The intensity may change, for example,because of dirt on the lens or change of a monitoring target etc.

The master camera 11 synchronizes the slave cameras 12, 13, 14 bytransmitting a synchronizing signal to them. The synchronizing signalcomprises a start pulse. The synchronizing signal indicates for thecameras 12, 13, 14 the time that is the synchronization moment at whichthe integration should be ended for capturing images 11′, 12′, 13′, 14′shown in FIG. 1 b . The synchronization moment may be, for example, atrailing edge or rising edge of the start pulse. And based on thisending time of integration and own integration time slave cameras 12,13, 14 and the master camera 11 itself calculate their own startingtimes for integration.

The master camera 11 further synchronizes the lighting device 16 by alight synchronizing signal indicating a switching on and off times forthe lighting device 16 by a light controlling pulse. The switching ontime is indicated in this embodiment by a rising edge of lightcontrolling pulse. The lighting device 16 is switched on beforeintegration of any of the cameras 11, 12, 13, 14 starts. The switchingoff time is indicated in this embodiment by a trailing edge of the lightcontrolling pulse and it defines the time when to switch off thelighting device 16 and corresponds the synchronizing moment of thecameras 11, 12, 13, 14. If there are more than one lighting devices, themaster camera 11 synchronizes them all by the light synchronizingsignals so that they all will be switched off at the definedsynchronization moment.

The web inspection system 10 also comprises an edge marking device 18for marking the edges of the web 15 at certain predetermined intervals,but the edge marking device 18 is an optional device in the webinspection system 10.

The transmitted synchronizing signal also comprises serial data. Theserial data field may comprise additional information for slave cameras12, 13, 14. This additional information can be used for combining imagesor it may be some other kind of information, for example, forcontrolling camera settings. The smart synchronization signal and thelight synchronizing signal are explained more precisely in context withFIG. 2 .

It is however possible, that there are embodiments wherein instead ofarea scan cameras, line cameras are used. In addition, it is alsopossible, that all cameras 11, 12, 13, 14 are so called slave camerasand an external synchronization unit is used for transmitting thesynchronizing signal to them and the light synchronizing signal to thelighting device 16.

FIG. 1 b shows the images 11′, 12′, 13′, 14′ captured by the cameras 11,12, 13, 14 of the web inspection system 10 of FIG. 1 a correspondingly.The capturing of images 11′, 12′, 13′, 14′ was synchronized by the smartsynchronization signal as explained above and therefore the images 11′,12′, 13′, 14′ are parallel and taken at the same distance from thebeginning of the web 15 in the moving direction. As can be seen, thereis no defect or corresponding in any of the images 11′, 12′, 13′, 14′.

FIG. 1 c shows a combined image 17′ formed from the images 11′, 12′,13′, 14′ of FIG. 1 b . And because the images 11′, 12′, 13′, 14′ wereparallel and aligned, all their information is included in the combinedimage 17′. The images 11′, 12′, 13′, 14′ are successfully combined basedon the location information sent by the synchronization signal.

FIG. 1 d shows the images 11″, 12″, 13″, 14″ captured by the cameras 11,12, 13, 14 of the web inspection system 10 of FIG. 1 a correspondingly.Capturing of the images 11″, 12″, 13″, 14″ were again synchronized byusing the smart synchronization signal as explained above in contextwith FIG. 1 a and therefore the images 11″, 12″, 13″, 14″ are againparallel and aligned transversely to the moving direction of the web 15.As can be seen, there is a defect A in the border area of the images 11″and 12″. The defect A is located so that a first part of the defect A isin the image 11″ and the second part is in the image 12″. The defect Ais thus a single error distributed over the area of the two adjacentimages 11″, 12″.

FIG. 1 e shows a combined image 17″ formed from the images 11″, 12″,13″, 14″ of FIG. 1 d . And because the images 11″, 12″, 13″, 14″ wereparallel and aligned, all their information is included in the combinedimage 17″ correctly and the defect A is shown in a correct place andcorrectly i.e. as a single defect.

FIG. 1 f shows the images 11′″, 12′″, 13′″, 14′″ of the cameras 11, 12,13, 14 of the web inspection system 10 of FIG. 1 a correspondingly. Butthese images 11′″, 12′″, 13′″, 14′″ are not captured by using the smartsynchronization method and its synchronization signal, but using asimple synchronization signal. The image capturing frequencies ofcameras 11, 12, 13, 14 are also varying i.e. do not remain the same andas predetermined for the cameras 11, 12, 13, 14. Therefore the adjacentimages 11′″, 12′″, 13′″, 14′″ are yet captured simultaneously, at thesame time, but because the simple synchronization signal did not sendlocation information or other information suitable to be used incombining phase for the cameras 11, 12, 13, 14, the images 11′″, 12′″,13′″, 14′″ are mixed and one of the images 12′″ has been misplaced atthe combining phase and it has ended up in a different combined imagethan the other images 11′″, 13′″, 14′″, as shown in FIG. 1 g . The otherimages 11′″, 13′″, 14′″ are in the combined image 17′″ a and themisplaced image 12′″ is in the combined image 17′″b. Cameras 11, 12imaged the same defect A as in FIG. 1 d , but now the first part andsecond part of the defect A are in two different combined images 17′″aand 17′″b. And if a defect map is formed based on these combined images17′″a and 17′″b and the defect A of the web 15 is tried to be correctedbased on the defect map, the correction will not succeed, because thedefect A is not indicated correctly in that defect map. Whereas, if adefect map is formed based on the image 1 e and the defect A is tried tobe corrected based on that defect map, the correction will succeed,because the defect A is indicated correctly and in a correct place inthat defect map.

FIG. 2 shows synchronization signals of a smart synchronization methodof a web inspection system according to an embodiment of the presentdisclosure. The synchronization signal shown above is thesynchronization signal 20 transmitted by a synchronizing device, forexample a master camera to slave cameras. The purpose of thesynchronization signal 20 is to synchronize all the cameras with eachother to capture images simultaneously so that integration of imagesends at the same time and also to provide additional information to beadded with the images.

The synchronization signal 20 comprises a start pulse 22, serial data 23and a pause 24 between them. The synchronization moment 25, when allcameras end integration and lights are switched off, is the trailingedge i.e. falling edge of the start pulse 22 i.e. the start pulse 22contains information indicating the synchronization moment 25 for theslave cameras. But it may also be a front edge i.e. rising edge of thestart pulse 22 that indicates the synchronization moment 25 instead ofthe trailing edge. The cameras calculate their own starting time ofintegration i.e. a time point where they start integration based on ownintegration time from the synchronization moment 25, common for allcameras. The length of the start pulse 22 may be, for example 30-80 μs,for example, 50 μs. For example, if the start pulse 22 indicates thatthe synchronization moment 25 is after 50 μs (i.e. the length of thepulse 22 is 50 μs, and an integration time of a first camera is 20 μs,the first camera starts the integration after 30 μs from starting of thestart pulse 22 i.e. 20 μs before the synchronization moment 25 and anintegration time of a second camera is 15 μs, the second camera startsthe integration after 35 μs from starting of the start pulse 22 i.e. 15μs before the synchronization moment 25.

After the end of the start pulse 22 and before the serial data 23 thereis the pause 24. The length of the pause 24 may be few μs, for example2-10 μs. The serial data 23 comprises additional information for theslave cameras. The information may be, for example, the exact end timeof the next light pulse i.e. the next synchronization moment, a lightingprofile number arranged to be used when capturing a next image, aposition information at the time of the next synchronization moment 25,an image number for the next image, a skip the next image flag, someapplication-specific data, checksum, etc. A suitable bit rate for a datafield of serial data 23 may be, for example, around 1 Mbit/s. Camerasmay add this additional information with captured image data when it istransmitted for an image data processing device and/or the cameras mayuse this additional information to control their own settings oroperations.

The signal shown below the smart synchronization signal 20 is a lightsynchronization signal 21 transmitted to lighting devices of the webinspection system arranged to illuminate the imaged area of the web alsoby the master camera. The light synchronization signal 21 indicatesswitching on and off times of lights of the lighting devices by lightcontrolling pulses 26. The trailing edge of the light controlling pulse26 indicates for the lighting devices a time point when to be switchedoff and this time corresponds the synchronization moment 25 of the smartsynchronization signal 20. Thus lighting devices are switchedsimultaneously with the time point when cameras end integration. Therising edge of the light controlling pulse 26 indicates for the lightingdevices a time point when to switch on and this time is before any ofthe cameras is configured to start integration. Or alternatively thetrailing edge of the light controlling pulse 26 indicates for thelighting devices the time point when to be switched on and the risingedge of the light controlling pulse 26 indicates for the lightingdevices the time point when to switch off.

FIG. 3 shows a web inspection system 30 using a smart synchronizationmethod according to an embodiment of the present disclosure forsynchronizing its cameras 31, 32, 33 and lighting devices 35, 36. Theweb inspection system 30 comprises a group of three cameras 31, 32, 33,two lighting devices 35, 36 and an image data processing device 31′,32′, 33′ for each camera 31, 32, 33. It is however possible that animage data processing device is common for one or more cameras. Thecameras 31, 32, 33 are arranged, adjacently i.e. side by side as a rowto expose image frames i.e. capture image matrixes from a web across itsentire width so that adjacent images together cover an areacorresponding the entire width of the web. One of the cameras 32 is amaster camera configured to act as a synchronizing device and the restof the cameras are slave cameras 31, 33.

The lighting devices 35, 36 are arranged to illuminate the area on theweb arranged to be imaged by the cameras 31, 32, 33. Switching off andon of the lighting devices 35, 36 is controlled by a lightsynchronization signal 38 transmitted by the master camera 32. The lightsynchronization signal 38 is explained in context with FIG. 2 .

Cameras 31, 32, 33 are arranged to capture images at a synchronizingmoment indicated by a single common smart synchronization signal 37. Atthe same synchronizing moment, the lighting devices 35, 36 are switchedoff. The master camera 32 transmits the smart synchronization signal 37to the slave cameras 31, 33. The smart synchronization signal 37 alsocomprises serial data comprising additional information for slavecameras 31, 33. The type of additional information is explained moreprecisely in context with FIG. 2 . Cameras 31, 32, 33 may use thereceived additional information for adjusting their settings for nextimage capturing. For example, they may receive a lighting profile numberarranged to be used when capturing a next image as additionalinformation and they may adjust their parameters based on thatinformation. Their parameters may be predefined for each lightingprofile number. Furthermore, after capturing of images, cameras 31, 32,33 may add the received additional information in context with thecaptured image data.

For example, if they have received a time of a synchronization moment, anumber of the next image, and/or a position in a moving direction at thetime of the synchronization moment as additional information, they mayadd this information to captured image data. After this, the cameras 31,32, 33 are arranged to transfer captured image data with additionalinformation to their image data processing devices 31′, 32′, 33′ forprocessing, analysis etc.

The image data processing devices 31′, 32′, 33′ may be an externaldevice. An image data processing device 31′, 32′, 33′ comprises at leastone processor, at least one memory including computer program code forone or more program units and means for receiving image data andadditional data wirelessly or via wired connection, for example, areceiver for receiving data wirelessly or via a wired connection. Theremay be multiple processors e.g. a general purpose processor and agraphics processor and a DSP processor, a Field-programmable gate array(FPGA) and/or multiple different memories e.g. volatile memory forstoring data and programs at run-time and nonvolatile memory like a harddisk for permanently storing data and programs. The image dataprocessing device 31′, 32′, 33′ may be any computing device suitable forhandling image data such as a computer. The image data processingdevices 31′, 32′, 33′ may be in electronic communication with thecameras 31, 32, 33 via signal lines, correspondingly. For handling thesignals to/from the signal lines, the image data processing device 32comprises I/O circuitry. The connection between the cameras 31, 32, 33and the image data processing devices 31′, 32′, 33′ may be a wired orwireless network. The image data processing devices 31′, 32′, 33′ mayalso include a video controller and an audio controller for generatingsignals that can be produced to the user with computer accessories. Thesimulator may produce output to the user through output means. The videocontroller may be connected to a display. The display may be e.g. a flatpanel display or a projector for producing a larger image. The audiocontroller may be connected to a sound source such as loudspeakers orearphones.

One of the image data processing devices 31′, 32′, 33′ may combineadjacent images as a combined image based on additional informationreceived in context with captured image data. The image data processingdevice 31′, 32′, 33′ may also analyse the combined image in order todetect defects or other events. Further, one of the image dataprocessing devices 31′, 32′, 33′ or an external computing device mayform a defect map, based on captured and combined images, indicatinglocation of the defects in the web with respect to the distancetravelled by the web so that correcting of those defects is easierbecause distances of defects from the beginning of the web can beaccurately shown. The defect map may be stored in a database anddisplayed.

The image data processing devices 31′, 32′, 33′ may also be a part ofthe cameras 31, 32, 33.

Cameras 31, 32, 33 form a first camera row, if there are more than onecamera rows i.e. at least one other camera row comprising at least oneslave camera, the same master camera 32 can be used for transmittingsmart synchronization signals for those slave cameras or each row maycomprise its own master camera that can be used for transmitting smartsynchronization signals for slave cameras of its own camera row. Thefirst camera row may also comprise more than shown three cameras 31, 32,33 as well as other possible camera rows. The number of cameras in onerow may depend on the width of the web and/or field of views of usedcameras.

The web inspection system 30 also comprises an edge marking device 34for marking the edges of the web at certain predetermined intervals, butthe edge marking device 34 is again an optional device in the webinspection system 30.

FIG. 4 shows an embodiment of the present disclosure, in which a blockdiagram of a synchronization method 40 of a web inspection system isdisclosed. The web inspection system is configured to monitor a movingweb and comprise a synchronizing device and at least one slave camera,and at least one lighting device arranged to illuminate an area of theweb arranged to be imaged by the cameras. In step 41 a synchronizingsignal is transmitted to the at least one slave camera by thesynchronizing device, wherein the synchronizing signal comprises atleast a start pulse and serial data comprising additional informationfor the at least one slave camera, and wherein all the cameras of theweb inspection system are configured to be synchronized with each otherbased on the synchronizing pulse indicating the synchronization momentwhen integration of cameras end. In step 42 a light synchronizing signalis transmitted to the at least one lighting device by the synchronizingdevice, wherein a light controlling pulse of the synchronizing signalindicates a switching on and off times of the at least one lightingdevice, which switching off time corresponds to the synchronizationmoment. And in step 43, a starting time of integration is calculatedbased on an individual integration time of a camera and thesynchronization moment common for cameras of the web inspection system.

The various embodiments of the present disclosure can be implementedwith the help of computer program code that resides in a memory andcauses an apparatus to carry out the present disclosure. For example,the apparatus that is a computing device, for example, an image dataprocessing device may comprise circuitry and electronics for analysing,receiving and transmitting data, a computer program code in a memory,and a processor which, when running the computer program code, causesthe apparatus to carry out the features of an embodiment. The processor,when running the computer program code may cause a web inspection systemcomprising a synchronizing device and at least one slave camera, and atleast one lighting device arranged to illuminate an area of the webarranged to be imaged by the cameras to carry out all the steps of thefollowing method: transmitting a synchronizing signal to the at leastone slave camera by the synchronizing device, wherein the synchronizingsignal comprises at least a start pulse and serial data comprisingadditional information for the at least one slave camera, and whereinall the cameras of the web inspection system are configured to besynchronized with each other based on the synchronizing pulse indicatingthe synchronization moment when integration of cameras end, transmittinga light synchronizing signal to the at least one lighting device by thesynchronizing device, wherein a light controlling pulse of thesynchronizing signal indicates a switching on and off times of the atleast one lighting device, which switching off time corresponds to thesynchronization moment, and calculating a starting time of integrationbased on an individual integration time of a camera and thesynchronization moment common for cameras of the web inspection system.

Considerable advantages are achieved by the present disclosure whencompared to methods and systems of existing web inspection systemscomprising at least two area scan cameras. By means of the arrangementaccording to the present disclosure it is possible to synchronize areascan cameras by a smart synchronizing method, wherein a smartsynchronizing signal is transmitted by a master camera that is one ofthe cameras of the web inspection system to other cameras, slavecameras, of the web inspection system. In addition, by means of thesmart synchronizing signal according to the present disclosure it isalso possible to transmit additional information to slave cameras andutilize the additional information not only for image capturing but alsowhen analysing the captured image data. In the smart synchronizingmethod also lighting device(s) of the web inspection system can becontrolled by light synchronization signal(s) for enabling best possibleimaging result.

It is obvious that the present disclosure is not limited solely to theabove-presented embodiments, but it can be modified within the scope ofthe appended claims.

1. A smart synchronizing method of a web inspection system formonitoring a moving web and comprising a master camera configured to actas a synchronizing device and at least one slave camera, and at leastone lighting device arranged to illuminate an area of the web arrangedto be imaged, the method comprising: transmitting a synchronizing signalto the at least one slave camera by the synchronizing device, whereinthe synchronizing signal comprises at least a start pulse and serialdata comprising additional information for the at least one slavecamera, which additional information is configured to be used forcombining of images of adjacent cameras, and wherein all the cameras ofthe web inspection system are configured to be synchronized with eachother based on the start pulse indicating a synchronization moment whenintegration of the cameras ends, transmitting a light synchronizingsignal to the at least one lighting device by the synchronizing device,wherein a light controlling pulse of the light synchronizing signalindicates switching on and off times of the at least one lightingdevice, which switching off time corresponds to the synchronizationmoment, and calculating a starting time of integration based on anindividual integration time of a camera and the synchronization momentcommon for the cameras of the web inspection system.
 2. The smartsynchronizing method according to claim 1, wherein the method furthercomprises: starting integration of the cameras at their calculatedstarting times of integration and switching on the at least one lightingdevice for illuminating the area on the web imaged by the cameras, andending integration of all the cameras and switching off the at least onelighting device at the time of the synchronization moment.
 3. The smartsynchronizing method according to claim 2, wherein the method furthercomprises: reading intensity values from pixels of the cameras measuredduring integration and resetting pixels, forming captured image databased on the read intensity values, and transmitting the captured imagedata with the received additional information to an image dataprocessing device.
 4. The smart synchronizing method according to claim3, wherein the method further comprises: forming a combined image fromthe received image data using the received additional information by theimage data processing device.
 5. The smart synchronizing methodaccording to claim 1, wherein all the cameras of the web inspectionsystem are area scan cameras.
 6. A web inspection system for monitoringa moving web and comprising a master camera as a synchronizing deviceand at least one slave camera, at least one lighting device, and animage data processing device, wherein the cameras are synchronized witheach other by using the method of claim
 1. 7. A web inspection systemaccording to claim 6, wherein the at least one slave camera is an areascan camera.
 8. A web inspection system according to claim 6, whereinthe cameras of the web inspection system are arranged adjacently as arow.
 9. A web inspection system according to claim 6, wherein the webinspection system further comprises an edge marking device.
 10. Acomputer program product, stored on a computer readable medium andexecutable in a computing device, wherein the computer program productcomprises instructions for a master camera acting as a synchronizingdevice of a web inspection system comprising at least one slave cameraand at least one lighting device to perform the method of claim 1.